The present invention relates to ignition systems for internal combustion engines and more particularly a semiconductor ignition system employing a magneto.
Known current interruption type transistorized ignition systems employing a magneto have a circuit construction in which a transistor and an ignition coil are connected in parallel with the generating coil of a magneto. The output of the generating coil is first short-circuited by the transistor and then the transistor is turned off at the time of ignition of the engine, thus utilizing the resulting transient voltage in the generating coil to cause a rapid current flow to the primary winding of the ignition coil and utilizing the resulting magnetic flux change to generate a high voltage in the secondary winding of the ignition coil.
However, in known ignition systems of the above type the base current is shunted to turn off the transistor. The base current must be reduced as far as possible since it represents a current which does not flow to the ignition coil or one which is ineffective in the generation of a high voltage. As a result, it has been necessary to use Darlington-connected power transistors having an amplification factor of about 100 times. The Darlington-connected power transistors are disadvantageous in that they are expensive and their collector-emitter saturation voltage is high. Another disadvantage is that, due to a high collector-emitter voltage generated upon turning the transistors on, in order to prevent this high voltage from causing current flow to the ignition coil before turning off of the transistors, a voltage-regulator element such as a diode must be connected between the transistors and the ignition coil, thus requiring a complicated and expensive circuit construction.